From biological treatment processes such as an activated sludge process, a large amount of sludge is generated, wherein organic wastewater such as sewage needs to be dealt with. It is a serious problem how to treat and dispose of the sludge. Conventionally, the sludge has been dewatered through a dehydrator by adding a dewatering assistant, then disposed of in a landfill or by incineration. However, in the case of disposing of sludge as in a landfill, there occurs a problem of a significant increase in disposal cost, owing to the shortage of available landfill area. In the case of incinerating sludge, there occurs several problems, which are, overloading of the incinerator and how to dispose the ash after incineration. Accordingly, the above-mentioned problems result in difficulties.
An anaerobic digestion (methane fermentation) process is known to the world as one of a method for reducing the volume of sludge. The process, however, needs a long retention time so that the size and capacity of a tank becomes gigantic. In addition, the effect, which is given by reducing the sludge, is not so high, even in the gigantic tank. Above all, the digested sludge, which is left after the digestion treatment, cannot help dewatering, before disposal treatment.
Various processes have been presented to reduce the volume of sludge, up to now. For example, Japanese Patent Laid-Open No. H9-253684 discloses a process to reduce the volume of generated sludge by solubilizing the withdrawn sludge in the anaerobic fermentation step, followed by recycling the fermented withdrawn sludge to the activated sludge system. Furthermore, there is a proposed technology to reduce the volume of generated excess sludge, which is applied to the wastewater-treatment technology. This technology conducts the steps of: withdrawing a part of the returned sludge; applying an alkali-treatment to the withdrawn and returned sludge by adding an alkali; applying biological treatment to the alkali-treated sludge under an anaerobic, anoxic, or microaerophilic condition; and recycling the biologically-treated sludge to the biological treatment step such as an aeration tank.
According to these conventional technologies, however, solubilizing the sludge merely by the anaerobic fermentation treatment needs a long retention time for attaining a sufficient solubilized ratio, which inevitably increases the size of the solubilization tank. For example, in the case that enzymes produced by thermophilic aerobic bacteria solubilize the sludge, the running cost increases, which is the heating cost and the running cost for running the aeration in order to maintain the aerobic condition.
As for the solubilization of sludge by ozone oxidation, foaming trouble in the ozone oxidation tank may occur, and a treatment system for emitting ozone is required. Furthermore, the investment cost to install an ozonizer and the running cost to add ozone is high. So, the initial investment cost increases to a great degree simultaneously with an increasing running cost. With regard to the solubilization by a hot-alkali-treatment process, a large amount of chemicals are required, which causes a cost-increase for the chemicals and the heating. This results in increasing the running cost.
As mentioned above, without applying a large scale of the solubilization tank and with the low running cost, no conventional technologies can achieve the volume reduction of sludge.